When the "cluster" option in the "bootstrap" command is specified, how
is the resampling implemented?

My guess is that resampling occurs only within clusters. Namely, that
given N observed data which constitute the sampe and r clusters in
which the N observations are grouped, r simultaneous resamplings occur
(each B times, number of replications specified) where the resampling
for the rth cluster uses only observations which are included in the
cluster. Am I right?